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118 Commission II Invited paper
Mathematical Methods in the Design
of Photogrammetric Plotters
by U. V. HELAVA
National Research Council
Ottawa, Canada.
1. Historical note.
The history of stereoscopic measuring instruments goes back about sixty years, during
which time many different designs were proposed and various plotters constructed. It is
not the author’s intention to review in detail the entire manifold development of this
particular type of instrument, but rather, to select significant advances in instrumental
development. A brief description of these wili provide the background for a presentation
of an outline of the newest designs which are classified as analytical plotters.
The forefather of all stereomeasuring instruments is the Pulfrich-Zeiss Stereocom-
parator built in 1901. Almost simultaneously and independently Fourcade arrived at a
similar solution. The Sterocompara-
Data flew when Stereocomparator is used tor, which has had a rennaissance
recently, may be used for the precise
measurement of coordinates and pa-
rallaxes on a stereopair. In order to
Viewing Computa- Final ; i :
Measuring H tions Co-ord. establish the shape and dimensions
ST } x of a photographed object, these coor-
2 | dinates must be processed by a con-
< Usually a time gap here | siderable amount of computation. No
a . . . .
= | plotting is possible for three reasons:
= | firstly, the computations are too
NN | time-consuming; secondly, the direc-
tion of data-flow in such a system
rator — — — -No Connection — À I
Operaio does not permit the decisions that
are needed in plotting, e.g., the loca-
Fig. la. tion of a contour line can be found
only by a trial and error procedure;
and thirdly, disturbing vertical parallaxes must be removed by hand.
The first stereophotogrammetric plotter was the Orel-Zeiss Stereoautograph built in
1908. This instrument attempted to solve the three difficulties limiting the use of the
stereocomparator for plotting purposes by using a mechanical analog computer. This
solution marks a significant step forward. The computations were performed fast enough
to allow continuous plotting, and the nature of the mechanical analogues used in the in-
strument made it possible to apply controls to the model coordinates, thus permitting the
necessary decisions to be made. However, the solution was good for only one special case
where the two camera axes were horizontal (the instrument was designed for terrestrial
photogrammetry) and parallel or symmetrically convergeant. If the two axes were not
perpendicular to the base, the viewing was disturbed by vertical parallaxes. For these
